Process for separating tin and arsenic compounds



Patented May 22, 1928.

UNITED STATES WILLIAM T. LITTLE, OF WESTFIELD, NEW JERSEY.

PROCESS FOR SEPARATING TIN AND ARSENIC OMPOUNDS.

No Drawing.

This invention relates to an improved method of separating chemicalcompounds having different 'meltmg points and solubilities.

The invention relates more particularly to a process for separating tincompounds from arsenic compounds when the two are present in stronglyalkaline solutions. Generally stated, the process in a pre ferred formthereof comprises efiecting the separation of soluble salts or other compounds by concentrating their solution while maintainin the temperatureabove the melting point'o one of them until precipitation of anothercontained in solution occurs, and washing the precipitated salt or saltsor other compounds with a washing liquor above the melting point of thesalt or salts or other compounds whose melting point has been exceeded.The principle embodied in the above process may also be applied inanother manner by concentrating the solution of mixed salts or othercompounds until a mixture of salts or other compounds is precipitatedand then washing the mixed salts or other compounds with a liquid hotenough to melt and take up and dissolve one of them while permitting theother or others toremain undissolved or only partly dissolved. Whenproceeding in the manner last indicated mixed solid salts or compoundsmay be used as the starting materials instead of solutions. v

The practice of the above processes is 111-. lustrated below by the searation of tin and arsenic from. strongly a ali ne solutions, assolutions of sodium h droxide. The starting material employe in thisspecific embodiment of the principle of my invention is a sodiumhydroxide solution comprising sodium stannate and sodium arsenate,although the practice of the process can be extended, as will beunderstood from the foregoing, to the separation of other substances. Invarious chemical processes solutions strongly alkaline with caustic sodaand containing arsenicand tin com ounds are obtained, audit is a furtherob ect of the present invention to obtain pure tin compounds from thesesolutions. Hitherto, the separa-' tion of the tin from the arseniccompounds has been conducted with indifferent success by one of the twofollowing methods.

In one method the solution is concentrated by boiling until it is at aspecific gravf Application filed November 27, 1925. Serial a... 71,838.

ity of approximately 1.2 andthen allowed to cool to atmospherictemperatures which allows the sodium arsenate to crystallize out as abulky precipitate of hydrated sodium arsenate. While theoretically notin is precipitated by this method, in practice it has been found thatthe arsenic precipitate carries a considerable amount of tin byabsorption which cannot be recovered by any. known method of washing.Therefore, this method involves the loss of considerable tin values.

In the second method of separation-the tin content of the solution isprecipitated by adding to the solution CO gas or a soluble bicarbonatesuch as sodium bicarbonate. In this process the tin is precipitated ashydrated oxide of tin in the form of a voluminous, finely dividedprecipitate. It is found that even with the greatest of care inprecipitating' and with extreme persistence in subsequent washing of theprecipitate, that the hydrated oxid of tin precipitated persists incontaining considerable amounts of arsenic, which renders the hydratedoxid of tin of an inferior quality.

. I have now discovered that if tin and arsenic compounds are broughtinto solutions strongly alkaline with caustic soda. and the sodiumstannate crystals which separate in such solutions are removed andWashed free from the mother liquor by means of wash water stronglyalkaline with caustic soda, while in the meantime the temperature iskept above about 855 centigrade, the melting point of sodium arsenate,then the sodium stannate crystals thus obtained will be practical] freefrom arsenic compounds. The solution strongly alkaline with soda andcontaining arsenic and tin is referably concentrated. by boiling until te free NaOH content is above 30 grams--preferably 35 to 40-per 100 cc.of solution. This serves to free practically all the tin from thesolution as sodium stannate crystals. These crystals arefiltered oif'hot and washed with a hot 'NaOH solution containing above 30grams-preferably 35 to 40-of sodium hydroxide per 100 cc. of solu tion.'Any KOH, solvent, or other liquid capable of issolving or melting outthe sodium arsenatc above its melting" point may be used to remove anyadhering mother liquor containing arsenic.

other alkali, as for exam le 10 The concentration of the alkali 11o usedfor washing may obviousl vary somewhat, but should always be su cient toprevent solution of the tin compounds. The temperature of the alkalishould be about or above 855 centigrade, the melting point of sodiumarsenate. When the solution from which the tin compounds have beenseparated is allowed to cool, the sodium arsenate separates at once ashydrated arsenate of soda and may be separated from the free N aOHpresent in the solution by filtration or otherwise.

My improved process of separation above described is not confined tosolutions ob-.

tained by concentration by boiling but may be applied to all mixtures oftin and arsenic salts in caustic soda solutions containing more thanabout 30 grams of free NaOH per 100 co, in whatsoever way such solutionsmay be obtained.

A modification of the process is to obtain the sodium stannate crystalsas described above but without the indicated extreme care to keep thecrystals at all times above about 85.5 centigrade, and then to wash thesodium stannate crystals which will be contaminated with sodium arsenatecrystals with a solution containing more than about 30 grams of NaOHper-100 cc. at a temperature above 85.5? centigrade. This hot solutionof caustic will melt and dissolve the sodium arsenate crystals but willnot appreciably dissolve the sodium stannate crystals.

The solution of arsenic and tin used above may also be concentrateduntil both sodium stannate and sodium arsenate are ,preci itated withoutregard to the regulation of t 1e temperature. Thereafter the mixedcrystals may be washed with sodium hydroxide of the concentrationindicated above and at a temperature about or above 85.5 centigrade. Theprocess of separation may likewise be applied to mixed tin and arseniccompounds, as sodium stannate and sodium arsenate, without first placingthese salts in solution. For example, the mixed solid, or mixed solidand melted salts or other compounds may be nate from sodium arsenatefrom a mixture including them which comprises treating the.mixture withan alkali metal hydroxide heated sufliciently to melt sodium arsenate.

3. The process of separating sodium stannate from sodium arsenate from amixture including them which comprises leaching the mixture with sodiumhydroxide heated suificiently to melt sodium arsenate.

4. The process of separating sodium stannate from sodium arsenate from amixture including them which comprises treating the mixture with asodium hydroxide solution at a temperature not lower than about 855 C.

5. The process of separating sodium stannate from sodium arsenate from amixture including them which comprises leachin the mixture with asolution of a strenfgt more than 30 grams of sodium hy roxide per 100cubic centimeters of solution and at a temperature above about 985 C.

6. The process of separating tin compounds from arsenic compoundspresent in an alkaline solution which comprises concentratin thesolution to the oint of precipitation o the tin com ound, t ereafterseparating the precipitate tin compound and washing it at a temperatureabove about 85.'5 C.

The process of separating tin compounds from arsenic compounds presentin an alkaline solution which comprises concentrating the solution untilthe tin separates as sodium stannate, and thereafter separating andwashing the sodium stannate with alkali solution above about 85.5 C.

In testimony whereof I aflix my signature.

WILLIAM T. LITTLE.

